It is customary at present to mount usual engine accessories which are driven by the propulsion engine of a bus or similar vehicle on mounting pads or brackets provided on the engine itself. These accessories are driven by V-belts or other power transmitting means from the crank shaft or cam shaft of the engine. Some prior attempts have been made to mount such engine accessories on a separate drive unit, for example, as shown in U.S. Pat. Nos. 3,263,663; 3,613,645 and 3,606,874. However, these prior accessory drive mechanisms are all more or less integrally and rigidly connected to the engine, so that engine vibration is transmitted to the drive unit and the accessories mounted thereon. Also, in some cases such drive units create an undesirable elongation of the engine assembly, and when the engine is removed for servicing or replacement it usually is necessary either to remove the accessory drive mechanism and accessories with the engine, or to unbolt the accessory assembly from the engine by an operation requiring a considerable amount of time.